An ultrasonic double-sheet detecting method is known to detect a conveyance of a plurality of paper sheets in which the paper sheets overlap each other. In the ultrasonic double-sheet detecting method, a threshold common to all paper sheets is calculated in advance, the threshold is automatically set in a detected-wave high-level setting circuit of a controller at the time of the start of a paper feeding unit, and the threshold is used to determine whether the number of paper sheets from the paper feeding unit is two or not.
Further, a double feed detecting circuit including a white reference storage unit and a reference value correcting unit is known. The white reference storage unit stores paper-sheet-penetration light amount value information that is measured by a sensor in a state where there is no paper sheet, the sensor being provided on a conveyance path for conveying a paper sheet. The reference value correcting unit calculates a reference value used in an electric signal converting process at the time of conveying a paper sheet. The reference value correcting unit compares white reference information from the white reference storage unit, with the paper-sheet-penetration light amount value information depending on a paper sheet thickness of a document to perform calculation. Thereby, the reference value correcting unit obtains a calculation result as a white reference change amount to correct an analogue-to-digital (A/D) conversion reference value. On the basis of the corrected A/D conversion reference value, an A/D conversion unit converts a penetration light amount measured value of a conveyed document into an electrical signal to be output as paper sheet thickness value information. A conveyance controller compares the paper sheet thickness value information with the double feed value information stored in a paper sheet thickness storage unit to perform calculation. Thereby, the conveyance controller detects the double feed of documents.
Furthermore, it is known that a paper-type detection unit that detects a paper-type of a document, and a paper thickness detection unit that detects a paper thickness are arranged between a separation unit and a conveyance roller. Both of the units detect a reflected light amount and a penetrated light amount. Information from two sensors is treated as reference data that is a penetrated light amount from the paper thickness detection unit corresponding to a reflected light amount from a paper-type detection unit. The double feed of documents is determined by the comparison of the information from the two sensors with the recorded reference data.
Related art is disclosed in Japanese Laid-open Patent Publications No. 05-193786, No. 06-9106 and No. 2003-137457.